Known EGR apparatuses for suppressing nitrogen oxide (NO.sub.x) emission from an internal combustion engine are divided into two types. In the first type of EGR apparatus, the exhaust gas is recirculated into the intake system of the engine at a position located upstream of the throttle valve. In the second type of EGR apparatus, the exhaust gas is recirculated into the engine intake system at a position located downstream of the throttle valve. In these known EGR apparatuses it is desired to recirculate a sufficient amount of exhaust gas for reducing to a great extent the amount of the NO.sub.x emission, due to the strict regulations directed against exhaust pollution from engines, which is undoubtedly a great public nuisance.
However, the above-mentioned first type EGR apparatus, in which the exhaust gas is recirculated into the intake system at the position upstream of the throttle valve, suffers from the shortcoming that a sufficient amount of exhaust gas can not be recirculated during low load engine operation. This is because, (1) in this type of EGR apparatus the exhaust gas is recirculated by a pressure of the exhaust gas due to the rotation of the engine, and (2) a sufficiently high exhaust gas pressure cannot be formed due to low rotational speed of the engine which is operating under the low load condition.
The above-mentioned second type of EGR apparatus, in which the exhaust gas is recirculated into the intake system at the position downstream of the throttle valve, suffers from the shortcoming that a sufficient amount of exhaust gas cannot be recirculated during high load engine operation in which the throttle opening is large. This is because, (1) in this type of EGR apparatus the exhaust gas is recirculated by a suctional force due to engine vacuum formed downstream of the throttle valve, and (2) a sufficiently large level of vacuum cannot be formed during the high load engine operation due to the large throttle opening.